1. Field of the Invention
The present invention relates in general to a backlight device having a pre-combined diffuser and substrate, thereby simplifying the process of assembling the backlight device in a LCD and scanner factory.
2. Description of the Related Art
Referring to FIG. 1, a conventional backlight device for a liquid crystal display (LCD) or a scanner includes a tubular lamp 11, a lighting guide 16 and a diffuser 15. The tubular lamp 11 emits light, and the light propagates in the lighting guide 16 and propagates out from the diffuser 15.
The lighting guide 16 includes a transparent substrate 14 and a white reflection sheet 12. A plurality of protrusions 13 are provided on the substrate 14 to scatter the light propagating in the substrate 14. The diffuser 15 is used for further scattering the light so that the distribution of the light out from the diffuser 15 is more uniform. If no diffuser is provided, the images of the protrusions 13 can be observed by the user in front of the backlight device.
Referring to FIG. 2, the light propagates in the transparent substrate 14 in two paths, indicated by reference numerals 17 and 18: (1) Light 17 is scattered by the protrusions 13 and then reflected by the reflection sheet 12 back to the substrate 14. Alternatively, light 17 is directly reflected at the interface between the protrusions 13 and the substrate 14. In either situation, light 17 is finally reflected back to the substrate 14. A large part of light 17 has a small angle of incidence, thus goes through the interface 19 and propagates out from the diffuser 15. A small part of light 17 has a large angle of incidence and is reflected at the interface 19 back to the substrate 14. (2) Light 18 directly propagates toward the interface 19. The light 18 has a large angle of incidence and is totally reflected at the interface 19.
In conclusion, the light reflected at the interface 19 including a small part of light 17 and the whole of light 18. The light reflected at the interface 19 is scattered by the protrusions 13, propagates through the substrate 14 and then propagates out from the diffuser 15.
It is noted that the light 18 does not propagate toward the protrusions 13 at the beginning. However, the light 18 is totally reflected at the interface 19 back to the substrate 14 and then is scattered by the protrusions 13. It is therefore understood that the light 17 and 18, which propagate in different paths, is scattered by the protrusions 13.
The diffuser 15 and the substrate 14 of the conventional backlight device are two separate elements. The LCD or scanner manufacturers need to buy the two elements separately from the suppliers and assemble them in the factories.